Metal step coverage has been of prime importance throughout the history of integrated circuit (IC) manufacture. Step coverage, however, has still been a major problem for IC manufacturers even into the late 1980s. Poor step coverage can be found at the sharp vertical step metal to substrate contacts, metal to metal vias, and metal crossovers. As dimensions shrink, conventional techniques used to improve step coverage fall short of expectations and are limited to stringent design criteria.
One method of improving step coverage has been realized through the use of thermal processing. Openings are cut through thick insulating layers using known dry etching processes. The sharp corners of the openings resulting from the dry etch process are rounded off by thermal processing at high temperatures. With the effect of surface tension, at temperatures close to melting point temperatures, almost all of the known insulating materials through which openings are formed tend to bead up eliminating sharp corners and reducing surface area. This reduction in surface area with no reduction in volume is a more thermodynamically stable arrangement.
The direct application, however, of thermal processing has certain disadvantages. For example, in the case of a metal contact, the contact resistance between metal and substrate is often reduced by the use of a suitable implant in the substrate. Thermal processing tends to nullify the effects of an implant because at melting point temperatures of reflow glass, the implanted species tend to diffuse out of the substrate and into the process ambient before the metal layer is deposited. When such diffusion into the process ambient occurs, a number of special implantation steps, according to the specific type of technology employed, may have to be added after thermal processing to achieve the desired contact resistance between metal and substrate.
Historically, an undoped silicon oxide layer of approximately 1500 Angstroms was grown between the insulating layer and the active areas. However, etching contact openings through insulating materials such as phosphorus silicate glass (PSG) or borophosphorus silicate glass (BPSG) may also etch the silicon dioxide layer exposing the underlying active areas. When the insulating layer is then reflowed, the dopants from the insulating layer tend to migrate to underlying active regions.
It would be desirable to use a metal oxide layer between the insulating layer such as a doped glass layer and the active area such that the metal oxide layer can be selectively etched over the insulating layer. It would be desirable to form the metal oxide layer to a sufficient thickness to prevent dopant diffusion from the insulating layer to the underlying active regions.